The present invention relates generally to the field of the motion detectors, and in particular, to a motion detector which can efficiently and cost-effectively suppress harmonic frequencies emanating from the motion detector.
It is a well known fact that motion detector units utilising the Doppler effect for motion detection radiate unwanted harmonic frequencies along with the main signal, or the fundamental frequency. Most countries require that any motion detection units being sold must comply with their requirements for harmonic suppression. The requirements vary from country to country. Some require that only the second harmonic be suppressed, while others who are more stringent, require suppression up to the third harmonic frequency.
The motion detector manufacturing industry has employed various ways to meet the requirements. At least one method is disclosed in U.S. Pat. No. 5,262,783 (U.K. Patent Application No. GB 2253108A).
Because a motion detector must meet various regulatory and commercial requirements, producing a commercially viable motion detector unit is not easy particularly in those countries where the suppression must reach the third harmonic. To address all of the concerns, not only must the motion detector meet the harmonic suppression requirement (which in itself is not easy) it must also prevent any undue degradation of the main signal such that the intended function of the motion detector is not compromised. Moreover, motion detector unit must be cost competitive.
As in many industries, cost is an important consideration when designing a motion detector unit. While it is not necessarily difficult to design a motion detector unit to meet the technical specification of a particularly country, it is often difficult to reduce the cost of the unit to a point which is commercially competitive. Hence, any motion detector design which can facilitate cost saving without causing performance degradation would be highly useful in the motion detector manufacturing industry.
Therefore, it is the general object of the present invention to overcome the shortcomings of the prior art motion detectors as mentioned above.
It is another object of the present invention to provide a cost-effective motion detector which can effectively suppress harmonic frequencies.
The present motion detector unit effectively suppresses harmonic frequencies and is cost-effective to build. The motion detector unit includes a main casing which houses the main microwave circuit board. Enclosing the casing and the circuit board is a cover. An antenna circuit board is attached to the bottom side of the main casing. The cover has partitioning walls at the bottom side of the cover which press against the main microwave circuit board when the cover is enclosed over the casing. The main circuit board has a plated bottom side and a top side which carries the microwave circuitry. The plated bottom side of the main circuit board makes electrical contact with the floor of the casing which is also made of conductive material such as EMI-shielded plastic or aluminum to provide a ground plane for the circuitry. The antenna board has a plated bottom side which makes electrical contact with the bottom side of the casing.
The circuit for the present motion detector includes a microwave oscillator for producing an oscillatory signal. In the preferred embodiment, the signal operates at an xe2x80x9cX-bandxe2x80x9d frequency in the range of 9.35 GHz to 10.687 GHz, though other frequencies are possible. The oscillator is coupled to a filter which is a low-pass filter with a relatively wide stop bandwidth. The filter is then coupled to a transmitting antenna which radiates a microwave signal to a target such as a human being. A power supply is routed through the transmission antenna and provides DC voltage to the microwave oscillator. The oscillator is further coupled to a mixer. The mixer is coupled to a filter which is in turn coupled to a receiving antenna. The receiving antenna receives the microwave signal which is reflected back from the target. The mixer mixes the original signal from the oscillator and the reflected signal received through the receiving antenna to generate an intermediate frequency (IF) which is sent to an output channel through the filter. The output IF signal can then be sent to an external circuitry for processing. Typically, an amplifier and a DC-blocking capacitor are required to interface with the output channel.
The main circuit board includes a plurality of plated grounding through-holes which are arranged in an elongated pattern which compartmentalize the circuitry. In this embodiment, there are four main compartments. The first compartment contains the circuitry for the microwave oscillator; the second compartment contains the circuitry for the filter; the third compartment contains the circuitry for the mixer; and the fourth compartment contains the circuitry for the filter.
The antenna circuit board comprises separate transmit and receive antenna arrays. Each antenna array comprises two patch elements which are connected to common feed striplines. The stripline for the transmit array is connected via a wire to the power supply. The stripline for the receive array is connected via a wire to the output channel which carries the intermediate frequency.
The bottom side of the cover has a set of partitioning walls which are arranged in a pattern to coincide with the pattern of the plated grounding through-holes. The partitioning walls generally define four compartmentalized cavities. When the cover is placed over the circuit board, the respective cavities enclose their respective compartments. In the preferred embodiment, the height of the partitioning walls is quarter wavelength of the operating frequency of the microwave oscillator.
When the cover is properly enclosed over the casing carrying the main circuit board, the partitioning walls should press firmly against the plated grounding through-holes to assist in providing a good contact between the circuit board and the plated floor of the casing. The partitioning walls assist in harmonic suppression by isolating the microwave circuitry to reduce any coupling of signals, and by shorting to ground any leakage of harmonic radiation which may occur.